1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the upgrading of catalytic-reformate quality.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Growing demand for high-purity aromatics as petrochemical intermediates also is a driving force for the upgrading of naphtha.
Catalytic reforming is a major focus, as this process generally supplies 30-40% or more of the gasoline pool and is the principal source of benzene, toluene and xylenes for chemical syntheses. Increased reforming severity often is accompanied by a reduction in reforming pressure in order to maintain yield of gasoline-range product from the reforming unit. Both higher severity and lower pressure promote the formation of olefins in reforming, and the 1-2+% of olefins in modern reformates contribute to undesirable gum and high endpoint in gasoline product and to particularly troublesome impurities in recovered high-purity aromatics streams.
Reformate and aromatics extracts recovered from reformate often are clay treated to polymerize the small amounts of olefin present [see, e.g., U.S. Pat. No. 3,835,037 (Fairweather et al.)]. This procedure, however, forms a heavy polymer which is an undesirable gasoline component which effects deposits in engines; further, disposal of the spent clay may be difficult and expensive. A problem facing workers in the art, therefore, is to discover a method of olefin removal which does not suffer the above drawbacks.
U.S. Pat. No. 5,658,453 issued to M. B. Russ et al. illustrates the use of an olefin selective saturation catalyst to selectively hydrogenate olefins present in a liquid phase stream recovered from reforming zone effluent stream. This process employs a limited and controlled hydrogen supply to promote selectivity for olefin saturation over aromatics saturation.
Considering selective hydrogenation of olefins, U.S. Pat. No. 3,869,377 (Eisenlohr et al.) teaches elimination of aliphatic unsaturates from a reformate by cooling a reaction mixture from hydroforming which contains hydrogen and aromatics and passing this mixture in gaseous state through a reactor containing a catalyst comprising oxides of Group 6 and/or 8 metals [preferably cobalt and molybdenum]. Russian disclosure SU1513014-A (Maryshev et al.) teaches hydrogenation of reforming products at a temperature of 150.degree.-250.degree. C. in the presence of aluminum-platinum catalysts. Selective hydrogenation of small quantities of alkenes in xylene-isomerization product, using a hydrogenation metal supported on a crystalline borosilicate molecular sieve, is disclosed in U.S. Pat. No. 5,015,794 (Reichmann).
Hydrogenation of olefins by adding a reactor within the hydrogen circuit of an associated unit suffers the disadvantage of adding pressure drop to the circuit, and also does not provide control of the ratio of hydrogen to olefin in the saturation zone.